I've been reading M. Thatcher's book The_Downing_Street_Years, and she spends a lot of time discussing the Westland affair. Her account seems completely orthogonal to the one given in the Wikipedia (in fact, they seem to focus on totally different aspects of this affair).
Why was this affair of such an importance? How was it covered in the media? Why did it pose problems at all to M.Thatcher's party? What prompted the resignations (it seems that no criminal or otherwise inappropriate activity happened at any point)?
I was asked to provide more background on the Westland affair, which is hard, since I do not understand it myself. But I will try to provide some essential details nevertheless.
It has to do with the financial rescue of a British company Westland Helicopters. The principal bidder for the rescue were jointly Sikorsky and Fiat, although the British Minister of Defense, Michael Heseltine,European consortium (it is unclear, which companies were involved, and whether they were involved with the same intensity). The opposite view (as far as I udnerstand) was advocated Trade and Industry Secretary Leon Brittan. Thatcher herself claims to be neutral, but critisizes Heseltine's behavior as unethical. In the end Heseltine had resigned.
Wikipedia focuses on the scandal related with disclosure of a confidential letter, having to do with the above mentioned rescue, which led to resignation of Leon Brittan.
It is unclear why this disclosure was so damaging and whether the dispute of American-led or European takeover of the Westland was of importance (Thatcher herself mentions the rise of anti-Americanism in the wake of the affair.)